The proposed research is a neuroethological approach to the study of vertebrate social communication. The system of choice is the highly sophisticated electric communication system known for two groups of freshwater fishes, the Mormyridae of Africa and the Gymnotiformes of South America. Both field and laboratory studies are planned. There are two main goals: the first is to define the structure of communication signals, the second, to uncover the mechanisms of signal processing in the electrosensory system relevant to the process of species recognition. The electrosensory system is an ideal system for studying temporal processing of rapid, time-varying waveforms. The studies will have relevance not just to the electrosensory system, which is specialized for temporal processing, but also to other similar sensory systems like the auditory system. Both mormyrid and gymnotiform fishes send and receive electric signals for electrolocation and for electrical communication. Both groups have species which produce brief, pulsatile Electric Organ Discharges (EODs) which have stereotyped, often species-specific, and in some cases sex-specific waveforms. Pulses are repeated at variable rates to make up characteristic sequences of pulse intervals (SPIs) used for communication. Behavioral experiments on the mormyrids have already demonstrated that electric fish attend to the temporal characteristics of these waveforms in species- and in sex-recognition. The new research will be directed at: 1) Defining communication signals encoded in the SPIs of mormyrids and gymnotiforms, and at determining the relative importance of SPIs and EODs in species- and sex-recognition among the gymnotiformes through the use of field observation and playback experiments. 2) Uncovering a physiological basis for the fine sensitivity to temporal cues in the electrosensory system of mormyrids by the use of electrophysiological and neuroanatomical techniques. Electrophysiological studies will concentrate on temporal processing of electric signals by the Knollenorgan electroreceptors of mormyrids, putative communication sensors for electric communication. Single unit electrophysiology and neuroanatomy will be used to trace the processing of EOD-like stimuli through this central pathway to the level of the midbrain.